A diesel particulate filter (hereinafter referred to as DPF) which traps particulate matter in exhaust gas discharged from a diesel engine, has the characteristic that pressure loss of the DPF increases or circulation resistance of the exhaust gas increases, as a result of an increase of a deposit amount.
If the pressure loss becomes higher than a predetermined value, it is determined that a reference value at which regeneration should be performed, has been reached, and regeneration of the DPF is performed wherein the particulate matter deposited in the DPF is burned and the DPF regenerates to a state in which it can trap again particulate matter.
Regeneration of the DPF is performed by raising the exhaust gas temperature by retarding a fuel injection timing of the diesel engine or performing a post-injection in the diesel engine. However, deposition of particulate matter in the DPF is not uniform, and there is a tendency for them to concentrate and deposit in the vicinity of the inlet of the DPF.
During regeneration, the particulate matter deposited near the inlet tend to burn with more difficulty as compared with particulate matter deposited further downstream from the DPF due to the following reasons.
In general, the DPF is cylindrical, and exhaust gas flows down the inside of the DPF along the center axis. If the exhaust gas temperature is raised for regeneration of the DPF, burning of particulate matter will occur first a little downstream from the front end of the DPF, and combustion of particulate matter will spread from there further downstream. Although particulate matter deposited downstream from the spark can use the combustion heat of upstream particulate matter for burning, particulate matter deposited near the inlet of the DPF must burn only with the heat of the exhaust gas.
Particulate matter deposited near the inlet of the DPF which cannot burn easily, but burns if sufficient regeneration time is allowed. The state where particulate matter deposited in the DPF are completely burned, is referred to as complete regeneration.
In a vehicle diesel engine, the vehicle running conditions change from moment to moment, and when the engine load is small, it is difficult to maintain the exhaust gas temperature of the diesel engine at a high level.
During regeneration of the DPF, if maintenance of exhaust gas temperature becomes difficult, regeneration of DPF is terminated at that time, and trapping of particulate matter by the DPF is resumed with some unburned particulate matter remaining mostly near the inlet.
The state where regeneration is terminated with some particulate matter remainder is referred to as a partial regeneration. This partial regeneration occurs more easily when the vehicle is driven in an urban area Particulate matter deposited near the inlet of the DPF induce further deposition of particulate matter, and the front end of the DPF becomes blocked. If the front end becomes blocked, pressure loss will be excessive as compared with the actual particulate matter deposit amount in the DPF. As a result, although the actual deposit amount has not reached the reference value for DPF regeneration, the regeneration treatment of the DPF is performed frequently and fuel consumption increases.